1. Field of the Invention (Technical Field)
The present invention relates to apparatuses and methods for electroplating and electrochemically modifying the surface finish of metal and semiconductor powders, particularly by continuous centrifugal means for encapsulation, anodizing, electroetching, electroforming, electrophoretic coating, electrosynthesis, and electrodeposition on powders without limitation on particle size, specifically including submicron- or nano-sized particles.
2. Background Art
The technologies for electrochemical enhancement of the surfaces of the particles in bulk powders has previously been limited to two main types: chemical copper and electrolytic nickel auto-catalytic processes; and rotary electroplating devices which require frequent stopping and starting of the electrolytic cell's rotation to tumble the powder to achieve uniform dispersion of the coating upon the particles. A limitation of the previous art using chemical or auto-catalytic processes is the cost of the chemical consumption due to the enormous surface areas of powders. Another limitation of known devices using the rotary techniques is the need to stop the cell to tumble the powder in order to disperse the coating and prevent agglomeration of the particles. Known devices of the latter type known in the art are typified by the disclosure of U.S. Pat. No. 5,879,520, the teachings of which are hereby incorporated by reference.
Previous rotary flow-through devices are capable of centrifugal clarification of the particles in solution and fixing them against the cathode ring for electrical contact. A disadvantage occurs, however, when rotation of the cell must be stopped to tumble the powder particles to foster even electrodeposition upon the individual particles. During this “stop phase,” the particles are re-suspended in the electrolyte solution. If the particles are of sufficient density, continuing the rotation of the cell re-clarifies the solution and again fixes the particles against the electrical contact ring, but the need periodically to stop and re-start cell rotation prolongs total processing times. Further, in the case of submicron-sized, low mass powders, the method of repeatedly stopping and resuming cell rotation is unacceptable from a practical standpoint, because the material particles remain in suspension (rather than in contact with the cathode) for impermissibly, nearly indefinite, lengths of time.
Also, laboratory experimentation and commercial application of the known rotary flow-through devices resulted in a determination that such devices have a powder particle size lower limit of approximately 20 micrometers for most common metals. These devices often have limitations related to the substrate powder's particle density, as well. Because previous rotary flow-through devices use a sintered membrane to allow the electrolyte to flow through the cell, a practical particle size limit occurs when the opening area of the sintered membrane must be smaller than the particle size. For powders below 50 micrometers mean particle diameter, the sintered membrane pores must be reduced to 25 micrometers. For powders below 20 micrometers, the sintered membrane pores must be 10 micrometers. When the sintered membrane pores are reduced below 10 micrometers, the discharge of electrolyte through the membrane is significantly impaired, which in turn depletes the ion species in the electrolyte, dramatically reducing the performance of the device. Because the distribution of size of the particles varies, it is possible to have particles smaller or equal in diameter to the openings in the sintered membrane, which in turn causes clogging or blinding of the membrane—further reducing performance. If the solution flow rate is increased to compensate for the ion depletion, the lightweight particles will overflow the cell, causing unwanted material loss and damage to the system.
Another problem with some previous rotary flow-through devices, such as the device of the U.S. Pat. No. 5,879,520, is that they require a complicated level control sensor to prevent the electrolyte solution from overflowing the top of the cell during the stop phase. This further limits the efficiency of solution flow, which also leads to ion depletion.
Further background in the field of rotary flow-through electroforming/electrodeposition devices and methods is supplied by U.S. Pat. Nos. 5,487,824 and 5,565,079, the disclosures of which are hereby incorporated by reference.
Moreover, each time the cell rotation is resumed (after stopping to tumble the substrate powder), time is required to clarify the solution and re-fix the particles to the face of the cathode ring; heavier particles are thrown into renewed contact with the cathode first, while finer particles require comparatively more time to move outward under centrifugal force. This results in heavier particles having preferential electrical contact with the cathode, resulting in a wide variance in the uniformity of the thickness distribution. In many cases, ultrafine particles will receive no electrodeposition at all.
Another limitation of known rotary flow-through devices is that the rectifier or power supply must be switched off and on in sync with the stopping and starting of the rotation of the cell. Besides causing extended process time during the off cycle, such intermittent voltage processes risk potential chemical damage to the substrate powder when no voltage potential is present.
Another limitation of known rotary flow-through device is the diameter and overall size of the cell, which had to be optimized to provide adequate stopping and starting performance. If the cell diameter is too large, the distance between the electrodes and the distance of travel of the particles became too great for efficient processing.
Another limitation of known rotary flow-through device is the required stop/start sequence means that the particles are fixed at the cathode during the on time, increasing the possibility of undesirably fusing or electroforming substrate components together. This obligates the high frequency stopping/starting to ameliorate agglomeration.
The foremost requirements for commercial electrodeposition apparatuses are to achieve cathode efficiency (e.g., 60-100 percent efficiency), prevent fusing or agglomeration of the particles, achieve high thickness uniformity, not corrode or damage the substrate powder, perform the electrodeposition in reasonable process time, and contain all particles in the apparatus with reasonable material handling methodologies.